Halftoning is a technique that simulates continuous tone imagery through the use of spots, varying either in size, shape or spacing. Halftoning exploits a property that the human visual system (HVS) is not as sensitive to high spatial frequency patterns. The pattern of the spots in halftoning is called the screen. In amplitude modulation (AM) halftoning, periodic cluster-dot screens are widely used for electro-photographic printers due to their print stability and low computational load and robustness to printer artifacts, but they are limited in angles and periodicity due to moiré patterns.
The traditional solution in the graphic arts and printing industry to deal with moirépatterns is to rotate identical square screens to angles that are maximally separated from each other. However, the effectiveness of this approach may be limited when printing with more than four colorants, i.e. N-color printing, where N>4. Moreover, accurately achieving the angles that have maximum angular separation may also be difficult for a digital printer, due to the relatively low resolution.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.